works like a charm, but I'd like to add automatically a link <a> element before the page break to help directing users to the following page.

I don't wish to put the link in the post content manually (from wp backend) nor have it inserted automatically in the post content (I don't want it saved along the post itself). Rather, I'd wish to have it generated just before <!--nextpage--> tag breaks the current page.

the filter is for content_save_pre and will convert via regex the more tags after the first one, if I add more filtering to content_save_pre it will be saved inside the post which I don't want; I think I like most the Krzysiek idea, very simple :)
–
nekojiraJun 23 '13 at 15:06